Exhaust Purifier for Internal Combustion Engine

ABSTRACT

An addition valve is provided upstream of an exhaust gas purifying catalyst (NOx catalyst) in an exhaust pipe and in the vicinity of a water jacket in a cylinder head, in addition to a fuel injection valve injecting fuel for combustion in a combustion chamber. The addition valve injects a reducing agent in an addition amount in accordance with an operation state of an engine. Using an engine coolant temperature correlated to a temperature of the addition valve, a target addition interval of the addition valve is made shorter with increase in the engine coolant temperature, so that the addition amount is increased (step  130 ). The temperature of the exhaust gas purifying catalyst (catalyst bed temperature) is raised as a result of increase in the addition amount. Here, a target injection amount of fuel, representing one of parameters other than the addition amount that affect the catalyst bed temperature, is restricted by using an injection amount upper limit, such that the catalyst bed temperature does not exceed an upper limit of an allowable range (steps  240, 250 ).

TECHNICAL FIELD

The present invention relates to an exhaust gas purifying apparatus ofan internal combustion engine, configured to provide an addition valveupstream of an exhaust gas purifying catalyst in an exhaust pipe forinjection of a reducing agent, in addition to a fuel injection valveinjecting fuel burnt in a combustion chamber of the internal combustionengine.

BACKGROUND ART

In an internal combustion engine operating by burning an air-fuelmixture at a high air-fuel ratio (lean atmosphere) in a wide operationregion, such as a diesel engine, generally, an NOx catalyst attaining afunction to purify a nitrogen oxide NOx in the exhaust gas is providedin an exhaust pipe thereof. As the NOx catalyst, for example, a catalystcarrying both an NOx absorbent capable of absorbing nitrogen oxide NOxin the presence of oxygen and a precious metal catalyst (precious metal)capable of oxidizing hydrocarbon HC on a honeycomb structure (carrier)made of porous ceramics is adopted.

The NOx catalyst has such a characteristic that it absorbs nitrogenoxide NOx in a state where oxygen concentration in the exhaust gas ishigh, while it emits nitrogen oxide NOx in a state where oxygenconcentration in the exhaust gas is low. In addition, if hydrocarbon HCor carbon monoxide CO is present in the exhaust gas when nitrogen oxideNOx is emitted in the exhaust gas, the precious metal catalyst promotesoxidation reaction of hydrocarbon HC or carbon monoxide CO, so thatoxidation-reduction reaction using nitrogen oxide NOx as an oxidationcomponent and using hydrocarbon HC and carbon monoxide CO as a reductioncomponent occurs therebetween. Namely, hydrocarbon HC and carbonmonoxide CO are oxidized to carbon dioxide CO₂ or water H₂O, andnitrogen oxide NOx is reduced to nitrogen N₂.

When the NOx catalyst absorbs a prescribed limit amount of nitrogenoxide NOx, the NOx catalyst absorbs no more nitrogen oxide NOx even in astate where oxygen concentration in the exhaust gas is high.Accordingly, in the internal combustion engine provided with such an NOxcatalyst in the exhaust pipe, an addition valve is provided upstream ofthe NOx catalyst in the exhaust pipe, separately from a fuel injectionvalve injecting fuel for combustion in the combustion chamber. Bysupplying a reducing agent such as light oil from the addition valve,nitrogen oxide NOx absorbed in the NOx catalyst is emitted and reducedand purified, so that NOx absorption capability of the NOx catalyst isrecovered and the NOx absorption amount of the NOx catalyst does notreach the limit amount (see, for example, Japanese Patent Laying-OpenNo. 2003-120392).

If the temperature of the addition valve is raised, however, a componentthat is likely to volatilize (hereinafter, referred to as a volatilecomponent) in the reducing agent that passes the addition valveevaporates, and a remaining deposit component is adhered and depositedin an injection hole of the addition valve and its surroundings. It maybe likely that the deposits clog the injection hole and the reducingagent is not appropriately injected from the injection hole. In order toaddress this problem, it is proposed to increase the amount of additionof the reducing agent from the addition valve under the condition thatthe temperature of the addition valve is high, such as when thetemperature of a coolant in the internal combustion engine is high andwhen the engine load is high.

When the amount of addition of the reducing agent is increased as above,the reducing agent at a low temperature passes the addition valve in alarge amount, and the reducing agent removes the heat of the additionvalve. The temperature at the tip end portion of the addition valveincluding the injection hole is lowered, and clogging of the injectionhole is suppressed. On the other hand, if the increased reducing agentburns on the exhaust downstream side of the addition valve, thetemperature of the NOx catalyst (catalyst bed temperature) is raised bythe heat generated during combustion. If the NOx catalyst is excessivelyheated, the catalyst bed temperature may exceed the upper limit of atemperature range (allowable range) where the NOx catalyst appropriatelyfunctions.

DISCLOSURE OF THE INVENTION

The present invention was made in view of such situations, and an objectof the present invention is to provide an exhaust gas purifyingapparatus of an internal combustion engine capable of suppressingoverheat of an exhaust gas purifying catalyst while suppressing cloggingof an injection hole of an addition valve.

A configuration for achieving the object above and a function and effectthereof will be described in the following.

The present invention is directed to an exhaust gas purifying apparatusof an internal combustion engine, configured to provide an additionvalve upstream of an exhaust gas purifying catalyst in an exhaust pipeconnected to a combustion chamber in addition to a fuel injection valveinjecting fuel for combustion in the combustion chamber of the internalcombustion engine, and to inject a reducing agent from the additionvalve in an addition amount in accordance with an operation state of theinternal combustion engine. The apparatus includes a control unitincreasing the addition amount with increase in at least one of atemperature of the addition valve and a value corresponding thereto andcontrolling a parameter other than the addition amount, that varies inaccordance with an operation of the internal combustion engine andaffects a catalyst temperature of the exhaust gas purifying catalyst,such that the catalyst temperature does not exceed an upper limit of anallowable range.

According to this configuration, the control unit increases the additionamount with the increase in the temperature of the addition valve andthe increased reducing agent passes the addition valve, so that the heatof the addition valve is removed. The temperature of the addition valveis lowered, evaporation of the volatile component in the reducing agentis suppressed, and clogging of the injection hole in the addition valveis suppressed. On the other hand, the increased reducing agent burns,and the temperature of the exhaust gas purifying catalyst is raised bythe heat generated during the combustion. In order to address this, thecontrol unit controls the parameter other than the addition amount thatvaries in accordance with the operation of the internal combustionengine and affects the temperature of the exhaust gas purifyingcatalyst. Therefore, even if the temperature of the exhaust gaspurifying catalyst is raised as a result of increase in the reducingagent, it is possible, by controlling the parameter, to suppress such asituation that the catalyst temperature of the exhaust gas purifyingcatalyst exceeds the upper limit of the allowable range.

Thus, according to the present invention, overheat of the exhaust gaspurifying catalyst can be suppressed by controlling the parameter, whileclogging of the injection hole of the addition valve is suppressed byincreasing the addition amount.

Preferably, when at least one of the temperature of the addition valveand the corresponding value is high, the control unit makes a degree ofincrease in the addition amount greater than when at least one of thetemperature of the addition valve and the corresponding value is low.

Here, as the amount of addition of the reducing agent injected from theaddition valve is greater, an amount of heat removed from the additionvalve by the reducing agent is increased and a degree of lowering in thetemperature of the addition valve becomes greater. In this regard, inthe present invention, when at least one of the temperature of theaddition valve and the value corresponding thereto is high, the degreeof increase in the addition amount is made larger than when at least oneof the temperature of the addition valve and the value correspondingthereto is low. In this manner, the degree of increase in the additionamount is varied in accordance with the temperature of the additionvalve. Therefore, when the temperature of the addition valve isrelatively low, excessive cooling of the addition valve due to excessiveincrease in the addition amount can be suppressed. In addition, when thetemperature of the addition valve is high, such a phenomenon as cloggingof the injection hole of the addition valve due to an insufficientaddition amount and resultant insufficient cooling of the addition valvecan be suppressed.

Further preferably, the addition valve is arranged in the vicinity of acoolant pipe provided in the internal combustion engine, and the controlunit employs a temperature of a coolant that flows through the coolantpipe as the corresponding value.

According to this configuration, as the addition valve is arranged inthe vicinity of the coolant pipe provided in the internal combustionengine, the temperature of the addition valve tends to be affected bythe heat of the coolant that flows in the coolant pipe. When thetemperature of the coolant is not so high, such as during a low orintermediate load operation or the like of the internal combustionengine, the addition valve is cooled as a result of heat removal by thecoolant, and clogging of the injection hole is unlikely. When thetemperature of the coolant is high, such as during a high load operationor the like of the internal combustion engine, efficiency in cooling theaddition valve is lowered, and the temperature of the addition valve mayexceed the temperature at which the volatile component in the reducingagent evaporates. Therefore, according to the present invention, theeffect of the invention described above is reliably obtained byemploying the temperature of the coolant as the value corresponding tothe temperature of the addition valve and by increasing the additionamount based on the corresponding value.

Further preferably, the control unit restricts an amount of fuelinjection from the fuel injection valve, as control of the parameter.

In the internal combustion engine and the exhaust gas purifyingapparatus, as the amount of fuel injection from the fuel injection valveis decreased, the temperature of the exhaust gas and the catalyst isaccordingly lowered. The injection amount is thus restricted as in thepresent invention, so that the injection amount is decreased as comparedwith a case where the injection amount is not restricted, and thetemperature of the exhaust gas is lowered. Then, the temperature of thecatalyst is lowered, and it is less likely that the upper limit of theallowable range is exceeded. In this manner, according to the presentinvention, the effect of the invention described above is reliablyobtained.

Further preferably, when at least one of the temperature of the additionvalve and the corresponding value is high, the control unit restrictsthe amount of fuel injection to a larger extent than when at least oneof the temperature of the addition valve and the corresponding value islow.

Here, as the injection amount is restricted to a larger extent, anamount of lowering in the temperature of the exhaust gas and thecatalyst as a result of restriction becomes greater. In this regard, inthe present invention, when at least one of the temperature of theaddition valve and the value corresponding thereto is high (when thedegree of increase in the addition amount is great and an amount ofincrease in the temperature of the catalyst is great), the injectionamount is restricted to a larger extent than when at least one of thetemperature of the addition valve and the value corresponding thereto islow. In this manner, the degree of restriction of the injection amountis varied in accordance with at least one of the temperature of theaddition valve and the value corresponding thereto, so that unduelowering in the engine output due to excessive restriction of theinjection amount can be suppressed when at least one of the temperatureof the addition valve and the value corresponding thereto is relativelylow. In addition, when at least one of the temperature of the additionvalve and the value corresponding thereto is high, such a situation thatrestriction of the injection amount is insufficient (the injectionamount is great) and the temperature of the catalyst consequentlyexceeds the upper limit of the allowable range can be suppressed.

Further preferably, the control unit decreases an amount of fuelinjection from the fuel injection valve, as control of the parameter.

In the internal combustion engine and the exhaust gas purifyingapparatus, as the amount of fuel injection from the fuel injection valveis decreased, the temperature of the exhaust gas and the catalyst isaccordingly lowered. The injection amount is thus decreased as in thepresent invention, so that the temperature of the exhaust gas is loweredas compared with a case where the injection amount is not decreased.Then, the temperature of the catalyst is lowered, and it is unlikelythat the upper limit of the allowable range is exceeded. In this manner,according to the present invention, the effect of the inventiondescribed above is reliably obtained.

Further preferably, when at least one of the temperature of the additionvalve and the corresponding value is high, the control unit decreasesthe amount of fuel injection by a larger amount than when at least oneof the temperature of the addition valve and the corresponding value islow.

Here, as the amount of fuel injection is decreased to a larger extent(the injection amount is decreased), an amount of lowering in thetemperature of the exhaust gas and the catalyst as a result of decreasebecomes greater. In this regard, in the present invention, when at leastone of the temperature of the addition valve and the value correspondingthereto is high (when the degree of increase in the addition amount isgreat and the amount of increase in the temperature of the catalyst isgreat), the injection amount is decreased more than when at least one ofthe temperature of the addition valve and the value correspondingthereto is low. In this manner, the degree of decrease in the injectionamount is varied in accordance with at least one of the temperature ofthe addition valve and the value corresponding thereto, so that unduelowering in the engine output due to excessive decrease in the injectionamount can be suppressed when at least one of the temperature of theaddition valve and the value corresponding thereto is relatively low. Inaddition, when at least one of the temperature of the addition valve andthe value corresponding thereto is high, such a situation that decreasein the injection amount is insufficient (the injection amount is great)and the temperature of the catalyst exceeds the upper limit of theallowable range can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of a first embodimentimplementing the present invention.

FIG. 2 is a schematic plan view showing an addition valve and a portionaround the same in an engine.

FIG. 3 is a timing chart illustrating a period during which a reducingagent is added and an interval of addition.

FIG. 4A is a flowchart showing a procedure for injection hole cloggingsuppression processing.

FIG. 4B is a flowchart showing a procedure for injection amountrestriction processing.

FIG. 5 is a characteristic diagram showing relation of an engine coolanttemperature, a catalyst bed temperature, an addition amount, and aninjection amount upper limit.

FIG. 6 is a flowchart showing a procedure for injection amount decreaseprocessing in a second embodiment of the present invention.

FIG. 7 is a characteristic diagram showing relation between an enginecoolant temperature and a correction amount.

FIG. 8 is a characteristic diagram showing relation of an engine coolanttemperature, a catalyst bed temperature, an addition amount, and acorrection amount.

BEST MODES FOR CARRYING OUT THE INVENTION First Embodiment

A first embodiment implementing the present invention will be describedhereinafter with reference to FIGS. 1 to 5. FIG. 1 shows a configurationof a multi-cylinder diesel engine (hereinafter, simply referred to as anengine) 11 serving as an internal combustion engine to which the presentembodiment is applied and an exhaust gas purifying apparatus 12. FIG. 2shows a schematic plan view of engine 11.

Engine 11 generally includes an intake pipe 13, a combustion chamber 14for each cylinder 10, and an exhaust pipe 15. An air cleaner 16purifying air taken in intake pipe 13 is provided in a most upstreamportion of intake pipe 13. In engine 11, an airflow meter 17 fordetecting a flow rate of air in intake pipe 13, a compressor 18A of aturbo charger 18, an intercooler 19, and an intake air throttle valve 21are sequentially arranged toward the intake air downstream side of aircleaner 16. Intake pipe 13 is branched at an intake manifold 22 providedon the intake air downstream side of intake air throttle valve 21, andconnected to combustion chamber 14 for each cylinder 10 through thebranch portion.

In a cylinder head 23 of engine 11, a fuel injection valve 24 injectingfuel for combustion in combustion chamber 14 is provided for eachcylinder 10. Each fuel injection valve 24 is supplied with fuel from afuel tank 26 through a fuel supply pipe 25. In fuel supply pipe 25, afuel pump 27 suctioning fuel from fuel tank 26 and pressurizing anddelivering the fuel and a common rail 28 serving as a high-pressure fuelpipe accumulating the delivered high-pressure fuel are provided. Fuelinjection valve 24 for each cylinder 10 is connected to common rail 28.

Meanwhile, a connection portion of exhaust pipe 15 and each combustionchamber 14 serves as an exhaust port 29. In exhaust pipe 15, an exhaustmanifold 31 for gathering the exhaust gas exhausted from each combustionchamber 14 through exhaust port 29 and a turbine 18B of turbo charger 18are provided.

In addition, engine 11 adopts an exhaust gas recirculation (hereinafter,referred to as “EGR”) apparatus 32 for recirculating a part of theexhaust gas in the intake air. EGR apparatus 32 includes an EGR pipe 33allowing communication between intake pipe 13 and exhaust pipe 15. Anupstream side of EGR pipe 33 is connected to a portion of exhaust pipe15 between exhaust manifold 31 and turbine 18B. In a midpoint of EGRpipe 33, an EGR cooler catalyst 34 purifying the recirculated exhaustgas, an EGR cooler 35 cooling the recirculated exhaust gas, and an EGRvalve 36 regulating a flow rate of the recirculated exhaust gas areprovided sequentially from the upstream side. The downstream side of EGRpipe 33 is connected to a portion of intake pipe 13 between intake airthrottle valve 21 and intake manifold 22.

In such engine 11, the air taken in intake pipe 13 is purified in aircleaner 16, and thereafter introduced in compressor 18A of turbo charger18. In compressor 18A, the introduced air is compressed and delivered tointercooler 19. The air of which temperature is raised as a result ofcompression is cooled in intercooler 19, and thereafter the air passesthrough intake air throttle valve 21 and intake manifold 22, anddistributed and supplied to combustion chamber 14 of each cylinder 10.The flow rate of the air in intake pipe 13 is regulated by controlling adegree of opening of intake air throttle valve 21. The flow rate of theair, that is, the amount of intake air, is detected by airflow meter 17.

In combustion chamber 14 into which the air is introduced, fuel isinjected from fuel injection valve 24 in the compression stroke of eachcylinder 10. Then, a mixture of the air introduced through intake pipe13 and the fuel injected from fuel injection valve 24 is burnt incombustion chamber 14. The combustion gas at high temperature and highpressure generated at this time causes a piston 37 to carry outreciprocating motion, a crankshaft 38 serving as an output shaftrotates, and drive force (output torque) of engine 11 is obtained. An NEsensor 39 detecting an engine speed NE indicating a rotation speed ofcrankshaft 38 is provided in engine 11.

The exhaust gas generated as a result of combustion in combustionchamber 14 of each cylinder 10 is introduced in turbine 18B of turbocharger 18 through exhaust manifold 31. When turbine 18B is driven bythe stream of the introduced exhaust gas, compressor 18A provided inintake pipe 13 is driven in synchronization, and the air is compressedas described above.

Meanwhile, a part of the exhaust gas generated as a result of combustionis introduced in EGR pipe 33. The exhaust gas introduced in EGR pipe 33is purified by EGR cooler catalyst 34 and cooled in EGR cooler 35, andthereafter recirculated in the air on the intake air downstream side ofintake air throttle valve 21 in intake pipe 13. The flow rate of theexhaust gas thus recirculated is regulated by controlling the degree ofopening of EGR valve 36.

Engine 11 is configured as described above. Exhaust gas purifyingapparatus 12 for purifying the exhaust gas exhausted from engine 11 willnow be described. Exhaust gas purifying apparatus 12 includes not onlyan addition valve 41 but also a plurality of (three) catalyticconverters (a first catalytic converter 42, a second catalytic converter43, and a third catalytic converter 44) serving as exhaust gas purifyingcatalysts.

First catalytic converter 42 at the most upstream portion is arranged onthe exhaust downstream side of turbine 18B. A storage-reduction type NOxcatalyst is accommodated in first catalytic converter 42. The NOxcatalyst is implemented, for example, in such a manner that a honeycombstructure is employed as a carrier and an NOx absorbent capable ofabsorbing nitrogen oxide NOx in the presence of oxygen and a preciousmetal catalyst (precious metal) capable of oxidizing hydrocarbon HC arecarried thereon.

The NOx absorbent has such a characteristic that it absorbs nitrogenoxide NOx in a state where the oxygen concentration in the exhaust gasis high, while it emits nitrogen oxide NOx in a state where the oxygenconcentration is low. In addition, if hydrocarbon HC, carbon monoxide COor the like is present in the exhaust gas when nitrogen oxide NOx isemitted in the exhaust gas, the precious metal catalyst promotesoxidation reaction of hydrocarbon HC or carbon monoxide CO, so thatoxidation-reduction reaction employing nitrogen oxide NOx as anoxidation component and employing hydrocarbon HC and carbon monoxide COas a reduction component occurs therebetween. Namely, hydrocarbon HC andcarbon monoxide CO are oxidized to carbon dioxide CO₂ or water H₂O, andnitrogen oxide NOx is reduced to nitrogen N₂.

Second catalytic converter 43 is arranged on the exhaust downstream sideof first catalytic converter 42. A storage-reduction type NOx catalystis accommodated in second catalytic converter 43. The NOx catalystincludes a porous material allowing passage of a gas component in theexhaust gas and preventing passage of particulate matter PM in theexhaust gas. This porous material is employed as the carrier of the NOxcatalyst, and the carrier carries the NOx absorbent and the preciousmetal catalyst. Third catalytic converter 44 is arranged on the exhaustdownstream side of second catalytic converter 43. In third catalyticconverter 44, an oxidizing catalyst purifying the exhaust gas throughoxidation of hydrocarbon HC and carbon monoxide CO in the exhaust gas iscarried.

Addition valve 41 is arranged in exhaust pipe 15, upstream of firstcatalytic converter 42. In the present embodiment, in order to satisfythis condition, addition valve 41 is attached to a portion in thevicinity of exhaust port 29 in cylinder head 23. Addition valve 41 isattached to cylinder head 23 in such a manner that an injection hole 41Aat the tip end is exposed in exhaust port 29. As shown in FIG. 2, thisposition is located around a water jacket 45 serving as a coolant pipeprovided in cylinder head 23. Addition valve 41 is attached to such aposition, so that addition valve 41 is cooled by the engine coolant thatflows through water jacket 45.

As shown in FIG. 1, addition valve 41 is connected to fuel pump 27through a fuel pipe 46, and injects and adds the fuel supplied from fuelpump 27 into the exhaust gas as the reducing agent. The added fueltemporarily turns the exhaust gas to a reduction atmosphere, so thatnitrogen oxide NOx stored in first catalytic converter 42 and secondcatalytic converter 43 is reduced and purified. In addition, secondcatalytic converter 43 simultaneously purifies particulate matter PM.

A coolant temperature sensor 47 detecting a temperature of the enginecoolant (engine coolant temperature THW) that flows through water jacket45 is attached to cylinder head 23. In addition, an exhaust gastemperature sensor 48 detecting a temperature of the exhaust gas(exhaust gas temperature) that passes a space between first catalyticconverter 42 and second catalytic converter 43 in exhaust pipe 15, thatis, the temperature of the exhaust gas before entering second catalyticconverter 43, is arranged in that space. Moreover, an exhaust gastemperature sensor 49 detecting a temperature of the exhaust gas thatpasses through a space downstream of second catalytic converter 43 inexhaust pipe 15, that is, the temperature of the exhaust gas immediatelyafter passing through second catalytic converter 43, is arranged in thatspace. Further, a differential pressure sensor 51 detecting adifferential pressure between the pressure of the exhaust gas on theexhaust upstream side of second catalytic converter 43 and the pressureof the exhaust gas on the exhaust downstream side thereof is arranged inexhaust pipe 15. In addition, oxygen sensors 52, 53 detecting aconcentration of oxygen in the exhaust gas are arranged on the exhaustupstream side of first catalytic converter 42 of exhaust pipe 15, andbetween second catalytic converter 43 and third catalytic converter 44,respectively.

An electronic control unit 61 serving as control means controls engine11 and exhaust gas purifying apparatus 12 described above. Electroniccontrol unit 61 includes a CPU executing various types of processinginvolved with control of engine 11, an ROM storing a program or datanecessary for control, an RAM storing a result of processing or the likeperformed by the CPU, an input/output port for transmitting/receivinginformation to/from the outside, and the like.

In addition to each sensor described above, an accelerator sensor 54detecting how far the accelerator is pressed down by a driver, a commonrail sensor 55 detecting an internal pressure (rail pressure) of commonrail 28, a throttle valve sensor 56 detecting a position of intake airthrottle valve 21, and the like are connected to the input port ofelectronic control unit 61.

Meanwhile, intake air throttle valve 21, fuel injection valve 24, fuelpump 27, addition valve 41, EGR valve 36, and the like are connected tothe output port of electronic control unit 61. Electronic control unit61 controls these components connected to the output port based on theresult of detection of each sensor, so as to control the operation ofengine 11, exhaust gas purification, and the like.

Electronic control unit 61 controls fuel injection, which represents onetype of control involved with the operation of engine 11. In fuelinjection control, a basic injection amount optimal for an operationstate of engine 11 is calculated based on an accelerator pressing-downdegree detected by accelerator sensor 54 and engine speed NE detected byNE sensor 39. In addition, a maximum injection amount is determined bycorrecting, based on signals from various sensors, the basic maximuminjection amount determined by engine speed NE (the theoreticallypossible injection amount). Comparing the basic injection amount and themaximum injection amount with each other, the smaller amount is set as atarget injection amount. In addition, basic target injection timing iscalculated based on the accelerator pressing-down degree and enginespeed NE, that are corrected based on a signal from various sensors, andtarget injection timing optimal for the operation state of engine 11 atthat time is calculated. Then, current supply to fuel injection valve 24is controlled based on the target injection amount and the targetinjection timing, so as to open/close fuel injection valve 24.

In addition, electronic control unit 61 controls the exhaust gaspurifying catalyst, which represents one type of control involved withpurification of the exhaust gas. In order to control the exhaust gaspurifying catalyst, four catalyst control modes, i.e., a catalystregeneration control mode, a sulfur poisoning recovery control mode, anNOx reduction control mode, and a normal control mode, are set, andelectronic control unit 61 selects the catalyst control mode inaccordance with a state of catalytic converters 42 to 44 and executesthat mode.

The catalyst regeneration control mode refers to a mode of control suchthat particulate matter PM deposited particularly in second catalyticconverter 43 is burnt and exhausted as carbon dioxide CO₂ and water H₂O.The sulfur poisoning recovery control mode refers to a mode of controlsuch that, when the NOx catalyst in first catalytic converter 42 andsecond catalytic converter 43 is poisoned with sulfur oxide SOx andstorage capability of nitrogen oxide NOx is lowered, sulfur oxide SOx isreleased.

The NOx reduction control mode refers to such a mode that nitrogen oxideNOx absorbed in the NOx catalyst is released and reduced and purified soas to recover the NOx absorption capability of the NOx absorbent, byadding and supplying the reducing agent to upstream of first catalyticconverter 42 in exhaust pipe 15 by means of addition valve 41 before theNOx absorption amount of the NOx absorbent in the NOx catalyst reachesthe limit.

For example, as shown in FIG. 3, in this mode, an open/close cycleconsisting of valve-opening and valve-closing of addition valve 41 isrepeated. The reducing agent is supplied from addition valve 41 as aresult of valve-opening, while supply is stopped as a result ofvalve-closing. By varying a period during which addition valve 41 isopen (addition period) and a time period from the start of valve-openinguntil the start of next valve-opening (addition interval), the amount ofaddition of the reducing agent is adjusted. In other words, as theaddition period is longer or as the addition interval is shorter, theaddition amount becomes greater. In the present embodiment, the additionamount is adjusted by varying the addition interval.

The state other than described above corresponds to the normal controlmode, in which the reducing agent is not added from addition valve 41.

Here, as injection hole 41A of addition valve 41 is exposed in exhaustport 29, the tip end portion of addition valve 41 including the portionaround injection hole 41A tends to be exposed to the exhaust gas and thetemperature thereof tends to be high. In addition, when the temperatureof the engine coolant (engine coolant temperature THW) that flowsthrough water jacket 45 is high in a high-load operation or the like ofengine 11, efficiency in cooling addition valve 41 by the engine coolantis lowered and the temperature of addition valve 41 tends to be high. Asthe temperature is raised, the volatile component contained in thereducing agent evaporates, and the remaining deposit component isadhered and deposited in injection hole 41A of addition valve 41 and itssurroundings. As the deposits clog injection hole 41A, the reducingagent may not appropriately be injected through injection hole 41A and aspray state may be poorer. In order to overcome such defects, it iseffective to moderately cool the tip end portion of addition valve 41,particularly the portion around injection hole 41A.

In the present embodiment, NOx reduction is controlled such thatclogging of injection hole 41A of addition valve 41 is suppressed. FIG.4A is a flowchart showing a specific procedure for clogging suppressionprocessing. Electronic control unit 61 executes a series of processingshown in the flowchart as processing to be performed every prescribedtime.

In the clogging suppression processing, initially in step 110,electronic control unit 61 calculates a target period of addition of thereducing agent, that is, a target valve-opening period of addition valve41, based on engine speed NE. In addition, in step 120, a targetinterval of addition of the reducing agent is calculated. Incalculation, a map defining in advance relation, for example, of enginespeed NE and the target injection amount with the target additioninterval is referred to. In the map, it is defined such that theaddition interval is shorter as engine speed NE is higher or as thetarget injection amount is greater. Then, the target addition intervalcorresponding to engine speed NE and the target injection amount at thattime is found based on the map. Here, as the target injection amount,the injection amount separately calculated in fuel injection controldescribed above is used.

In succession, in step 130, the target addition interval calculated instep 120 above is corrected, using the temperature of addition valve 41or a value corresponding thereto. Here, engine coolant temperature THWdetected by coolant temperature sensor 47 is employed as the valuecorresponding to the temperature of addition valve 41. This is becauseaddition valve 41 is arranged in the vicinity of water jacket 45provided in cylinder head 23 as described above, and the temperature ofaddition valve 41 tends to be affected by the heat of the engine coolantthat flows through water jacket 45. In correction, the target additioninterval in step 120 is corrected such that, when engine coolanttemperature THW is high, the target addition interval is made shorterthan when engine coolant temperature THW is low, that is, such that thenumber of times of addition per unit time is greater and the additionamount is increased.

In step 140, current supply to addition valve 41 is controlled based onthe target addition period in step 110 and the target addition intervalcorrected in step 130. As a result of current supply, addition valve 41is opened/closed, and the reducing agent is injected through injectionhole 41A into exhaust port 29. After the processing in step 140, aseries of clogging suppression processing ends.

As described above, as engine coolant temperature THW is higher, theefficiency in cooling of addition valve 41 by the engine coolant islowered and the temperature of addition valve 41 is raised. Meanwhile,as a result of increase in the addition amount, a large amount ofreducing agent not yet much affected by the heat of the exhaust gas andat a low temperature passes addition valve 41. The reducing agent thatpasses removes a large amount of heat of addition valve 41, and thetemperature of addition valve 41 becomes lower than the temperature atwhich the volatile component in the reducing agent evaporates.Consequently, formation of deposits in injection hole 41A and theportion around the same resulting from evaporation of the volatilecomponent is suppressed.

In addition, in increasing the addition amount, when engine coolanttemperature THW is high, the degree of increase is made larger than whenengine coolant temperature THW is low. In other words, the degree ofincrease in the addition amount is varied in accordance with enginecoolant temperature THW correlated with the temperature of additionvalve 41. Therefore, when the temperature of addition valve 41 (enginecoolant temperature THW) is relatively low, the degree of increase inthe addition amount is small, and the amount of heat removed fromaddition valve 41 by the increased reducing agent is relatively small.Accordingly, excessive cooling of addition valve 41 by the increasedreducing agent is less likely. Alternatively, when the temperature ofaddition valve 41 (engine coolant temperature THW) is high, the degreeof increase in the addition amount is great, and the amount of heatremoved from addition valve 41 by the increased reducing agent isgreater. Accordingly, such a phenomenon that increase in the additionamount is insufficient and addition valve 41 is not sufficiently cooledis less likely.

On the other hand, heat generated as a result of combustion of thereducing agent increased in the above-described manner causes increasein the temperature of the NOx catalyst (catalyst bed temperature) infirst and second catalytic converters 42, 43. If the NOx catalyst isexcessively heated, the upper limit of a temperature range (allowablerange) where the NOx catalyst appropriately functions may be exceeded.

In the present embodiment, in order for the catalyst bed temperature notto exceed the upper limit of the allowable range due to the increase inthe amount of addition of the reducing agent, control for restrictingthe amount of fuel injection from fuel injection valve 24 is carriedout. FIG. 4B is a flowchart showing a specific procedure for injectionamount restriction processing. Electronic control unit 61 executes aseries of processing shown in the flowchart as processing to beperformed every prescribed time.

In the injection amount restriction processing, initially in step 210,electronic control unit 61 reads engine coolant temperature THW detectedby coolant temperature sensor 47 at that time. Then, in step 220,electronic control unit 61 determines whether engine coolant temperatureTHW in step 210 is higher than a threshold value a that has been set inadvance. Threshold value ax represents an upper limit, or a value closethereto, of a range of temperature that engine coolant temperature THWmay take on the condition that the catalyst bed temperature does notexceed the upper limit of the allowable range even if the reducing agentin an amount increased in accordance with the increase in engine coolanttemperature THW is added.

Here, it is assumed that a determination condition in step 220 issatisfied (THW>α). Then, if the fuel is injected in accordance with thetarget injection amount, the catalyst bed temperature may exceed theupper limit of the allowable range due to the heat of the exhaust gas,the heat generated during combustion of the reducing agent, and thelike. Accordingly, in order to lower the temperature of the exhaust gassuch that the catalyst bed temperature does not exceed the upper limit,in step 230, the injection amount upper limit in accordance with enginecoolant temperature THW in step 210 is set. When engine coolanttemperature THW is high, the injection amount upper limit is set to avalue lower than when engine coolant temperature THW is low. That is,when deviation from threshold value α of engine coolant temperature THWis great, the injection amount upper limit is set to a value lower thanwhen deviation is small.

In succession, in step 240, whether the target injection amountcalculated separately in fuel injection control described above isgreater than the injection amount upper limit in step 230 is determined.If this determination condition is satisfied (target injectionamount>injection amount upper limit), the injection amount upper limitis set in step 250 as the final target injection amount to be instructedto addition valve 41. That is, the target injection amount is restrictedby the injection amount upper limit. After the processing in step 250, aseries of injection amount restriction processing ends.

In contrast, if the determination condition in step 220 above is notsatisfied (THW≦α) and if the determination condition in step 240 is notsatisfied (target injection amount≦injection amount upper limit), inboth cases, it is considered that there is no possibility that thecatalyst bed temperature exceeds the upper limit of the allowable range.Therefore, a series of injection amount restriction processing endswithout performing the processing in steps 230 to 250 in the formercase, or without performing the processing in step 250 in the lattercase. In these cases, the target injection amount is not restricted butused as it is as the final target injection amount.

Therefore, as engine coolant temperature THW is higher, through theclogging suppression processing described above, the amount of additionof the reducing agent from addition valve 41 is increased in order tosuppress clogging of injection hole 41A, and in addition, when enginecoolant temperature THW is high, the degree of increase is made largerthan when engine coolant temperature THW is low, whereby the catalystbed temperature is raised. On the other hand, if engine coolanttemperature THW exceeds threshold value a and if the catalyst bedtemperature is likely to exceed the upper limit of the allowable range,the injection amount upper limit is set based on engine coolanttemperature THW. The target injection amount is restricted so as not toexceed the injection amount upper limit. Namely, when the targetinjection amount exceeds the injection amount upper limit, the targetinjection amount is substantially decreased. As a result of restrictionof the target injection amount, the target injection amount is madesmaller than when it is not restricted, and the fuel in an amount inaccordance with that target injection amount is injected and burnt,whereby the temperature of the exhaust gas is lowered. As the increasein the catalyst bed temperature due to the exhaust gas is made smaller,the catalyst bed temperature is less likely to exceed the upper limit ofthe allowable range.

In restricting the target injection amount, when engine coolanttemperature THW is high, the injection amount upper limit is set to avalue lower than when engine coolant temperature THW is low, and thetarget injection amount is restricted to a larger extent. In otherwords, the degree of restriction of the target injection amount isvaried in accordance with engine coolant temperature THW used forcorrecting the target addition interval (increase in the additionamount). Therefore, when engine coolant temperature THW is relativelylow, that is, when the degree of increase in the addition amount isrelatively small and when the increase in the catalyst bed temperaturedue to the increase in the addition amount is small, the degree ofrestriction of the target injection amount is small. Accordingly, such aphenomenon that the target injection amount is excessively restrictedand output of engine 11 is unduly lowered is less likely. In addition,when engine coolant temperature THW is high, that is, when the degree ofincrease in the addition amount is great and when the increase in thecatalyst bed temperature due to the increase in the addition amount isgreat, the degree of restriction of the target injection amount isgreat. Accordingly, such a phenomenon that restriction of the targetinjection amount is insufficient (the target injection amount is greaterthan an appropriate value) and the catalyst bed temperature exceeds theupper limit of the allowable range is less likely.

FIG. 5 shows relation of engine coolant temperature THW, the catalysttemperature, the amount of addition of the reducing agent, and theinjection amount upper limit. Here, in a temperature region where enginecoolant temperature THW is lower than threshold value α, restriction ofthe target injection amount by the injection amount upper limit is notcarried out. In addition, in this temperature region, in order tosuppress clogging of injection hole 41A of addition valve 41, whenengine coolant temperature THW is high, the amount of addition of thereducing agent is made larger than when engine coolant temperature THWis low. With the increase in the addition amount, that is, with theincrease in engine coolant temperature THW, the catalyst bed temperatureis also raised and approaches to the upper limit of the allowable range.

When engine coolant temperature THW attains to threshold value α orhigher, the injection amount upper limit in accordance with enginecoolant temperature THW is set. When the target injection amount exceedsthe injection amount upper limit, the injection amount upper limit isset as the target injection amount, so that the target injection amountis made smaller, the temperature of the exhaust gas is lowered, and thecatalyst bed temperature is accordingly lowered. As a result of suchlowering, such a situation that the catalyst bed temperature exceeds theupper limit of the allowable range is suppressed. As deviation from theupper limit of the allowable range of the catalyst bed temperature isgreater, further increase in the addition amount is permitted.

On the other hand, due to the increase in engine coolant temperatureTHW, clogging of injection hole 41A is more likely. If the additionamount is increased after engine coolant temperature THW attains to aprescribed value β (>α), clogging of injection hole 41A is suppressed.In contrast, the catalyst bed temperature is raised with the increase inthe addition amount and approaches the upper limit of the allowablerange. In order to address this, the injection amount upper limit is setto a lower value with the increase in engine coolant temperature THW. Asa result of restriction based on the injection amount upper limit, thetarget injection amount is made smaller, and the temperature of theexhaust gas and the catalyst bed temperature are lowered.

According to the present embodiment described in detail above, thefollowing effects can be obtained.

(1) With the increase in the temperature of addition valve 41, theamount of addition of the reducing agent is increased, and the targetinjection amount representing a parameter other than the addition amountthat affects the catalyst bed temperature is controlled such that thecatalyst bed temperature does not exceed the upper limit of theallowable range. Therefore, while suppressing clogging of injection hole41A by increasing the addition amount, overheat of the NOx catalyst canbe suppressed by controlling the target injection amount.

(2) Engine coolant temperature THW is employed as the valuecorresponding to the temperature of addition valve 41. When enginecoolant temperature THW is high, the degree of increase in the additionamount is made larger than when engine coolant temperature THW is low.By thus varying the degree of increase in the addition amount inaccordance with engine coolant temperature THW, when the temperature ofaddition valve 41 is relatively low, excessive cooling of addition valve41 due to excessive increase in the addition amount can be suppressed.In addition, when the temperature of addition valve 41 is high, cloggingof injection hole 41A due to insufficient addition amount and resultantinsufficient cooling of addition valve 41 can be suppressed.

(3) The injection amount upper limit in accordance with engine coolanttemperature THW is set. When the target injection amount exceeds theinjection amount upper limit, the injection amount upper limit is set asthe final target injection amount. As a result of such restriction usingthe injection amount upper limit, the target injection amount is madesmaller than when no restriction is imposed, and the temperature of theexhaust gas is lowered. Accordingly, the catalyst bed temperature islowered and less likely to exceed the upper limit of the allowablerange. Therefore, the effect described in (1) above is reliablyattained.

In addition, as a result of restriction above, the target injectionamount can be decreased only when the catalyst bed temperature is likelyto exceed the upper limit of the allowable range, that is, only whennecessary. Therefore, unnecessary decrease in the target injectionamount can be suppressed, as compared with a case where the targetinjection amount is decreased without exception when engine coolanttemperature THW exceeds threshold value α.

(4) When engine coolant temperature THW is high, the injection amountupper limit is set to a value lower than when engine coolant temperatureTHW is low, whereby the target injection amount is restricted to alarger extent. By thus varying the degree of restriction of the targetinjection amount in accordance with engine coolant temperature THW,undue lowering in the output of engine 11 due to excessive restrictionof the target injection amount can be suppressed when engine coolanttemperature THW is relatively low. In addition, when engine coolanttemperature THW is high, such a situation that restriction of the targetinjection amount is insufficient (the target injection amount is great)and the catalyst bed temperature exceeds the upper limit of theallowable range can be suppressed.

(5) The temperature of addition valve 41 is affected not only by enginecoolant temperature THW but also by the heat of the exhaust gas. Thetemperature of the exhaust gas tends to vary in accordance with the fuelinjection amount and tends to lower as the fuel injection amount issmaller. Here, in the first embodiment, with the increase in enginecoolant temperature THW, the target injection amount is restricted anddecreased. Therefore, not only by increasing the addition amount butalso by decreasing the target injection amount, the temperature ofaddition valve 41 is lowered and the clogging phenomenon of injectionhole 41A can further effectively be suppressed.

Second Embodiment

A second embodiment implementing the present invention will now bedescribed with reference to FIGS. 6 to 8. The second embodiment isdifferent from the first embodiment in that the target injection amountis decreased, instead of restricting the same, for control such that thecatalyst bed temperature does not exceed the upper limit of theallowable range with the increase in the addition amount of the reducingagent. As the configuration of engine 11 and exhaust gas purifyingapparatus 12 is the same as in the first embodiment, description thereofwill not be repeated.

FIG. 6 is a flowchart showing a specific procedure for injection amountdecrease processing. Electronic control unit 61 executes a series ofprocessing shown in the flowchart as processing to be performed everyprescribed time.

In the injection amount decrease processing, initially in step 310,electronic control unit 61 reads engine coolant temperature THW detectedby coolant temperature sensor 47 at that time. Then, in step 320,electronic control unit 61 determines whether engine coolant temperatureTHW in step 310 is higher than threshold value α. Threshold value α isthe same as described in the first embodiment.

Here, it is assumed that a determination condition in step 320 above issatisfied (THW>α). Then, if the fuel is injected in accordance with thetarget injection amount as it is, the catalyst bed temperature mayexceed the upper limit of the allowable range due to the heat of theexhaust gas, the heat generated during combustion of the reducing agent,and the like. Accordingly, the processing for lowering the temperatureof the exhaust gas such that the catalyst bed temperature does notexceed the upper limit is performed. Specifically, in step 330, acorrection amount (>0) for decreasing the target injection amount iscalculated based on engine coolant temperature THW in step 310. Forexample, as shown in FIG. 7, the correction amount can be set such thatthe correction amount is smaller when engine coolant temperature THW islow, while the correction amount is greater as engine coolanttemperature THW is higher, that is, as deviation from threshold value αof engine coolant temperature THW is greater.

In succession, in step 340, in controlling fuel injection describedabove, the correction amount in step 330 is subtracted from the targetinjection amount calculated in a different routine, and the subtractionresult is set as the final target injection amount to be instructed tofuel injection valve 24. After the processing in step 340, a series ofinjection amount decrease processing ends.

In contrast, if the determination condition in step 320 above is notsatisfied (THW≦α), it is considered that there is no possibility thatthe catalyst bed temperature exceeds the upper limit of the allowablerange even though the addition amount is increased with the increase inengine coolant temperature THW. Therefore, in this case, a series ofinjection amount decrease processing ends without performing theprocessing in steps 330 and 340. Here, the target injection amount isnot corrected but used as it is as the final target injection amount.

Therefore, as engine coolant temperature THW is higher, through theclogging suppression processing described above, the amount of additionof the reducing agent from addition valve 41 is increased in order tosuppress clogging of injection hole 41A, and in addition, when enginecoolant temperature THW is high, the degree of increase is made largerthan when engine coolant temperature THW is low, whereby the catalystbed temperature becomes higher. On the other hand, if engine coolanttemperature THW exceeds threshold value α and if the catalyst bedtemperature is likely to exceed the upper limit of the allowable range,the correction amount is calculated based on engine coolant temperatureTHW. As a result of correction using the correction amount, the targetinjection amount is decreased. As the fuel in the decreased targetinjection amount is injected and burnt, the temperature of the exhaustgas is lowered. As the increase in the catalyst bed temperature due tothe exhaust gas is made smaller, it is less likely that the catalyst bedtemperature exceeds the upper limit of the allowable range.

In decreasing the target injection amount, when engine coolanttemperature THW is high, the correction amount is set to a value greaterthan when engine coolant temperature THW is low, and the targetinjection amount is corrected to a lower value. In other words, thedegree of decrease in the target injection amount is varied inaccordance with engine coolant temperature THW used for correcting thetarget addition interval (correction for increase in the additionamount). Therefore, when engine coolant temperature THW is relativelylow, that is, when the degree of increase in the addition amount isrelatively small and when the increase in the catalyst bed temperaturedue to the increase in the addition amount is small, the degree ofdecrease in the target injection amount is small. Accordingly, such aphenomenon that the target injection amount is excessively decreased andthe output of engine 11 is unduly lowered is less likely. In addition,when engine coolant temperature THW is high, that is, when the degree ofincrease in the addition amount is great and when the increase in thecatalyst bed temperature due to the increase in the addition amount isgreat, the degree of decrease in the target injection amount is great.Accordingly, such a phenomenon that decrease in the target injectionamount is insufficient (the target injection amount is greater than anappropriate value) and the catalyst bed temperature exceeds the upperlimit of the allowable range is less likely.

FIG. 8 shows relation of engine coolant temperature THW, the catalystbed temperature, the amount of addition of the reducing agent, and thecorrection amount for the target injection amount. Here, in atemperature region where engine coolant temperature THW is lower thanthreshold value α, the target injection amount is not decreased. Inaddition, in this temperature region, in order to suppress clogging ofinjection hole 41A of addition valve 41, when engine coolant temperatureTHW is high, the degree of increase is made larger than when enginecoolant temperature THW is low, so that the amount of addition of thereducing agent becomes greater. With the increase in the additionamount, that is, with the increase in engine coolant temperature THW,the catalyst bed temperature is also raised and approaches to the upperlimit of the allowable range.

When engine coolant temperature THW attains to threshold value α orgreater, the correction amount (>0) in accordance with engine coolanttemperature THW is calculated, and the target injection amount isdecreased by that correction amount. When fuel in an amount inaccordance with the decreased target injection amount is injected fromfuel injection valve 24 and burnt, the temperature of the exhaust gas islowered, and the catalyst bed temperature is accordingly lowered. As aresult of such temperature lowering, such a situation that the catalystbed temperature exceeds the upper limit of the allowable range issuppressed. As deviation between the catalyst bed temperature and theupper limit of the allowable range becomes greater, further increase inthe addition amount is permitted.

On the other hand, due to the increase in engine coolant temperatureTHW, clogging of injection hole 41A is more likely. If the additionamount is increased when engine coolant temperature THW attains to aprescribed value β (>α), clogging of injection hole 41A is suppressed,whereas the catalyst bed temperature is raised with the increase in theaddition amount and approaches the upper limit of the allowable range.In order to address this, a larger value for the correction amount isset, and the target injection amount is decreased to a larger extentusing this larger correction amount. When fuel in an amount inaccordance with the decreased target injection amount is injected fromfuel injection valve 24 and burnt, the temperature of the exhaust gasand the catalyst bed temperature are lowered.

According to the second embodiment described above as well, in additionto the effects similar to (1), (2) and (5) described above as in thefirst embodiment, the following effect can be obtained.

(6) The correction amount in accordance with engine coolant temperatureTHW is set, so that the target injection amount is decreased using thatcorrection amount. As a result of injection and combustion of the fuelin the decreased target injection amount, the temperature of the exhaustgas is lowered. Therefore, such a situation that the catalyst bedtemperature exceeds the upper limit of the allowable range can reliablybe suppressed by lowering the catalyst bed temperature.

(7) When engine coolant temperature THW is high, the correction amountis greater than when engine coolant temperature THW is low. By thusvarying the correction amount used for decreasing the target injectionamount in accordance with engine coolant temperature THW, when enginecoolant temperature THW is relatively low, undue lowering in the outputof engine 11 due to excessive decrease in the target injection amountcan be suppressed. In addition, when engine coolant temperature THW ishigh, such a situation that decrease in the target injection amount isinsufficient (the target injection amount is great) and the catalyst bedtemperature exceeds the upper limit of the allowable range can besuppressed.

The present invention can be implemented in another embodiment shownbelow.

-   -   A substance other than the fuel may be injected from addition        valve 41 as the reducing agent.    -   The amount of addition of the reducing agent from addition valve        41 may be adjusted by varying a period during which the reducing        agent is added, instead of or in addition to the addition        interval above.    -   In the embodiments, the addition amount is corrected by        correcting the target addition interval based on engine coolant        temperature THW, because the temperature of addition valve 41 is        affected by the engine coolant. Instead or in addition, the        addition amount (addition interval, addition period) may be        corrected based on a parameter other than engine coolant        temperature THW that affects the temperature of addition valve        41. For example, the temperature of the exhaust gas or the        engine load may be employed as the parameter. In this case, the        addition amount is increased as the temperature of the exhaust        gas is higher or as the engine load is higher. Alternatively,        the addition amount (addition interval, addition period) may be        corrected based on the detected temperature of addition valve 41        itself.    -   The parameter that is varied along with the operation of engine        11 and affects the catalyst bed temperature may include        supercharge pressure, injection timing, rail pressure, and the        like, in addition to the above-mentioned fuel injection amount.        Therefore, any of these parameters (including the target        injection amount) or combination thereof may be used to carry        out control such that the catalyst bed temperature does not        exceed the upper limit of the allowable range with the increase        in the addition amount. For example, an amount of air supplied        to combustion chamber 14 is increased by raising the supercharge        pressure, so that the temperature of the exhaust gas and hence        the catalyst bed temperature can be lowered. In addition,        earlier injection timing is set (injection timing is advanced)        so that the exhaust gas is emitted while the combustion pressure        is low, and the temperature of the exhaust gas (catalyst bed        temperature) can be lowered. Moreover, the end of an injection        period is moved ahead by raising the rail pressure, so that the        temperature of the exhaust gas (catalyst bed temperature) can be        lowered.    -   Exhaust gas purifying apparatus 12 of the present invention is        applicable to an engine in which addition valve 41 is arranged        at a position distant from water jacket 45 of cylinder head 23,        for example, on the exhaust downstream side of exhaust port 29.        In this case, as addition valve 41 is less likely to be affected        by the heat of the engine coolant, whether or not the        temperature of addition valve 41 exceeds the temperature at        which the volatile component in the reducing agent evaporates is        monitored based on a parameter other than engine coolant        temperature THW, and the amount of addition of the reducing        agent is increased based on the result of monitoring. Examples        of such parameters include the temperature of the exhaust gas,        the engine load, and the like as described above.

It should be understood that the embodiments disclosed herein areillustrative and non-restrictive in every respect. The scope of thepresent invention is defined by the terms of the claims, rather than thedescription above, and is intended to include any modifications withinthe scope and meaning equivalent to the terms of the claims.

1. An exhaust gas purifying apparatus of an internal combustion engine,configured to provide an addition valve upstream of an exhaust gaspurifying catalyst in an exhaust pipe connected to a combustion chamberin addition to a fuel injection valve injecting fuel for combustion inthe combustion chamber of the internal combustion engine, and to injecta reducing agent from said addition valve in an addition amount inaccordance with an operation state of the internal combustion engine,comprising: a control unit increasing said addition amount with increasein at least one of a temperature of said addition valve and a valuecorresponding thereto and controlling a parameter other than saidaddition amount that varies in accordance with an operation of saidinternal combustion engine and affects a catalyst temperature of saidexhaust gas purifying catalyst, such that said catalyst temperature doesnot exceed an upper limit of an allowable range.
 2. The exhaust gaspurifying apparatus of an internal combustion engine according to claim1, wherein when at least one of said temperature of said addition valveand said corresponding value is high, said control unit makes a degreeof increase in said addition amount greater than when at least one ofsaid temperature of said addition valve and said corresponding value islow.
 3. The exhaust gas purifying apparatus of an internal combustionengine according to claim 2, wherein said addition valve is arranged invicinity of a coolant pipe provided in said internal combustion engine,and said control unit employs a temperature of a coolant that flowsthrough said coolant pipe as said corresponding value.
 4. The exhaustgas purifying apparatus of an internal combustion engine according toclaim 1, wherein said control unit restricts an amount of fuel injectionfrom said fuel injection valve, as control of said parameter.
 5. Theexhaust gas purifying apparatus of an internal combustion engineaccording to claim 4, wherein when at least one of said temperature ofsaid addition valve and said corresponding value is high, said controlunit restricts said amount of fuel injection to a larger extent thanwhen at least one of said temperature of said addition valve and saidcorresponding value is low.
 6. The exhaust gas purifying apparatus of aninternal combustion engine according to claim 1, wherein said controlunit decreases an amount of fuel injection from said fuel injectionvalve, as control of said parameter.
 7. The exhaust gas purifyingapparatus of an internal combustion engine according to claim 6, whereinwhen at least one of said temperature of said addition valve and saidcorresponding value is high, said control unit decreases said amount offuel injection by a larger amount than when at least one of saidtemperature of said addition valve and said corresponding value is low.